The present invention relates to the field of electron emitters and in particular to a Schottky electron emitter having high transmission.
Focused electrons beams are used in many applications, for observing and processing microscopic features. Electrons are emitted from an electron source and then formed into a beam by electron optical elements, which are analogous to optical elements used for light but use electric or magnetic fields instead of curved glass surfaces. Electrons in the beam impact a workpiece and can induce charges in the workpiece of the point of impact. The beam also causes secondary electrons to be emitted from the workpiece at the point of impact and the secondary electrons can be collected and used to form an image of the work piece.
The number of electrons in the beam passing a particular point each second is referred to as the beam current. There are some focused electron beam (xe2x80x9ce-beamxe2x80x9d) applications, such as e-beam lithography and defect detection, in which very high beam currents are desirable. High beam currents result in significant increases in throughput, that is, the number of work pieces that can be processed in a given time period. Significant increases in throughput are desirable for present generation of e-beam tools and such increases will be a necessity for subsequent generations of electron beam tools.
An ideal, high current electron source would emit a large number of electrons, all having approximately the same energy, into a small angle cone around the optical axis. The number, angle, and energy of the electrons from an ideal source would remain stable over a long period of time. Real electron sources, however, emit electrons at various angles to the optical axis. The electron optics can only converge into the beam electrons that are emitted within a small cone around the optical axis into the beam, so not all of the electrons that are emitted end up as part of the beam. The electrons that are not transmitted in the beam add to the power consumption of the system without being useful. The ratio of the electron current in the beam to the total electron current that is produced by the emitter is referred to as the xe2x80x9ctransmissionxe2x80x9d of the emitter and is typically a few thousandths.
Transmission can be improved by concentrating the electron emission into a small solid angle around the beam axis. One common measurement of the angular concentration of electrons from a source is its angular intensity, Ixe2x80x2, defined as the number of electrons emitted per second per solid angle, typically measured in milliamperes per steradian (mA/sr). The angular intensity thus reflects not only the number of electrons emitted, but also the angular concentration of the electrons.
The most common high angular intensity electron source currently used in fine focus applications is a Schottky emitter. A Schottky emitter emits electrons from an emitter tip typically composed of tungsten and coated with zirconium and oxygen. The zirconium and oxygen reduce the amount of energy, referred to as the xe2x80x9cwork function,xe2x80x9d required to extract electrons from the emitter tip surface. The energy to extract electrons is provided in part by an electron current that heats the tip and in part by an electric field, referred to an extractor field, that provides additional energy to pull the electrons from the heated tip. Schottky emitters provide low energy spread, high stability, low noise, and long life. The conventional Schottky emitter in modem commercial systems typically operates at a maximum angular intensity of 1.0 mA/sr.
As shown in FIG. 1, the angular intensity of a modern Schottky source can be increased by increasing the total number of electrons emitted, for example by increasing the extraction field, which exponentially increases the number of electrons emitted. As shown in FIG. 2, however, the transmission decreases as the extraction current field is increased. Together, FIGS. 1 and 2 show that increasing the extraction field causes more electrons to be emitted, but a decreasing percentage of those additional electrons actually are in the central electron beam. Thus, the increased angular intensity comes at a cost of reduced efficiency.
The total emission current determines the maximum angular intensity that is practically achievable in a reliable emitter because it limits the maximum angular intensity at which the emitter can practically be operated. Because the power generated by the source is equal to the exponentially increasing emission current multiplied by the extraction voltage, the power generated increases exponentially along with the total emission current. It is preferable to have the total emission current below 750 xcexcA as excess current and/or power may have a detrimental effect, on the performance and stability of the electron column. Therefore, to operate at high angular intensity it is necessary to have a high transmission.
Traditionally the method to increase transmission for a Schottky emitter (of approximately the same cone angle) is to increase the radius of the emitter tip. FIG. 3 shows a graph of transmission versus the radius of the emitter tip. FIG. 3 shows that the transmission can be maximized by using a radius of approximately 1.1 xcexcm. Because the transmission peaks at about 1.1 xcexcm, increasing or decreasing the radius of a conventional Schottky emitter cannot further increase the transmission. Thus the conventional Schottky emitter source is limited, even with the radius optimized, to a total angular intensity of less than about 1.4 mA/sr.
Currently there are commercially available Zr/O/W Schottky emitters with high transmission. These high transmission sources are similar to conventional Schottky emitters, but differ in the shape of the tip and perhaps in the Zr/O coverage. The Zr/O coverage differences are difficult to determine, but affect the work function. A conventional Schottky emitter tip, as shown in FIG. 4, is typically produced by electrochemically etching with alternating current to produce an emitter tip with an end radius of between 0.3 xcexcm and 2.0 xcexcm and a cone angle of 17 to 25 degrees. U.S. Pat. No. 5,449,968 to Terui et al. for a xe2x80x9cThermal Field Emission Cathodexe2x80x9d describes a typical process for producing a conventional Schottky emitter.
A typical commercially available high transmission source has a shape characteristic of a direct current electrochemical etch with end radius of less than 0.5 xcexcm and cone angle between 11 and 20 degrees. The combination of a long narrower cone angle and small radius results in a higher field strength at the tip of the emitter and smaller emission area than the conventional Schottky emitter source for a given extraction voltage and tip geometry. The localized high field and small emission area result in these emitters operating with transmissions several times greater than a conventional Schottky emitter source. Some commercially available emitters claim to have an angular intensity of up to 8.0 mA/sr with total currents of less than 600 xcexcA, although applicants were unable to produce these high angular intensity levels using commercial available sources at the total current levels claimed by their manufacturers.
Moreover, the commercially available high angular intensity emitters have been found to be extremely inconsistent in their emission characteristics. The extreme inconsistency causes these emitters to typically operate, not with high transmission, but with transmission similar to a conventional Schottky emitter source. Even the emitters that do have high transmission often have other problems, including fluctuations in the beam current and/or total current, short term instability of the currents over a period of hours, long term drift of the currents over a period of days, and asymmetrical angular emission distribution. The significant inconsistency in transmission combined with the significant and frequent performance problems of the emitters that do have high transmission, makes the currently available commercial high Ixe2x80x2 emitters unpractical for use in a commercial focused e-beam system.
An object of the invention is to provide a process for producing an electron source having an accurately produced tip shape and suitable work function.
Another object of the invention is to provide an electron emitter having an accurately produced tip shape and suitable work function.
Another object of the invention is to provide an electron beam system using an electron emitter having an accurately produced tip shape and suitable work function.
The invention provides a method of producing electron sources, electron sources produced thereby, electron beam instruments using such sources, electron sources produced thereby, and a method of producing electron beam instruments using such sources. Applicants have found that a cause of problems in prior art electron sources is inconsistent emitter geometry and work function, often caused during the manufacturing step that lowers the work function of the emitter tip surface. By using a tip shaping process that produces a tip having desirable geometric characteristics and a work function lowering step that does not degrade the tip geometry, applicants produce an improved electron source.
In a preferred embodiment, applicants produce the desired emitter tip geometry by electroetching the tip, depositing a work function lowering material on the emitter, and then diffusing the work function lowering material to emitter tip. The prior art required an extended heating cycle to induce a chemical change in the deposited material before it would function as a work function lowering material. In the preferred embodiment, the deposited material is heated primarily to diffuse it to the emitter tip and not primarily to induce a chemical change, thereby reducing the duration and/or temperature of the heating step and consequently the alteration of the emitter tip geometry. The diffusion step is optionally performed with an electric field applied to the emitter tip, which field further reduces deformation of the tip geometry during heating.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.